8-Oxoguanine Glycosylase

8-oxogunanine glycosylase (hOGG1) is a DNA glycosylase coded from the OGG1 gene in humans; however, many homologs exist in different organisms and this enzyme was originally discovered in yeast. It is responsible for removing genotoxic lesions caused by oxidative damage in the presence of reactive oxygen species (ROS). Glycosylases, in general, are key enzymes for base excision repair and therefore are essential for maintaining integrity of the genetic material. Once a lesion is successfully excised, the transcription machinery of the cell can repair the DNA strand; however, if it is not repaired mutagenesis occurs possibly leading to cancer and other degenerative diseases.

=Function= hOGG1 repairs 7,8-dihydro-8-oxogunanine (8-oxoG, GO); this lesion arises from oxidative attack by ROS on G. It is a particularly dangerous, and stable mutation because GO can Hoogsteen base-pair with adenine causing G:C to T:A tranversions. hOGG1 is able to cleave the N-glycosylic bond between the deoxyribose moiety and GO leaving an apurinic-apyrimdinic (AP) site. It also has the intrinsic ability to cleave the 3’ phosphodiester of the AP site by β-elimination, acting as an AP lyase, and making it a bifunctional glycosylase. hOGG1 has greater affinity for GO when it is complementary to C, and hOGG1 also has catalytic activity towards other lesions such as formamidopyrimidines.

=Structure= hOGG1, a monomer, belongs to a super family of DNA repair enzymes that share a conserved, two-domain fold containing a helix-hairpin-helix DNA binding motif followed by a Glycine/Proline rich stretch and a invariant Aspartate. This motif is necessary for interacting with DNA to recognize and catalyze the substrate. As well, OGG1 is associated with two calcium ions that help stabilize the deformed DNA back bone at the site of the extruded lesion.

=Mechanism= Only 50,000 hOGG1 molecules protect the entire 6,000,000,000 nuclear base-pairs in a diploid cell. For this reason it is obvious that hOGG1 must have an efficient mechanism to catalyze GO and discriminate between GO and G even though they differ by only two atoms at C8 and N7.

Recognition
hOGG1 is able to discriminate GO from G with the help of a single hydrogen bond between a main chain carbonyl O of G42 and the protonated N7 position of the GO. Additional structural studies have indicated that the GO is extruded from the DNA helix and inserted deeply into a catalytic pocket where residues lining the pocket can directly interact to excise the lesion.

Catalysis
To excise GO, Lys 249 acts as a nucleophile attacking the C1' carbon in a SN1 reaction, when GO is inserted into the catalytic pocket. Then hOGG1 AP lyase activity uses a conserved lysine residue as a nucleophile to generate a covalently linked enzyme-DNA adduct that undergoes a series of subsequent transformations resulting in DNA strand exscission on the 3’ side of the lesion.

=Importance of hOGG1= Oxidative mutations in DNA are heavily implicated in aging and cancer so hOGG1 activity is necessary for many organisms to increase longevity. In fact C:G to T:A tranversions, that can be casued by GO, are very common in tumor suppressor genes and human cancers. GO has even been suggested as an indicator for breast cancer where ROS can accumulate. Mutations in glycosylase genes, like hOGG1, have also been linked to increased mutation rates.

=Additional Resources= For additional information, see: Cancer

3D structures of 8-Oxoguanine Glycosylate
DNA glycosylate

=References=